This Program entitled "Anti-viral Immune Responses in Lymph Nodes" seeks to gain a deeper understanding of the induction and regulation of immunological events that are elicited by viral antigens in lymph nodes (LNs). Many viruses and most conventional vaccines enter the body through the skin and travel via lymphatics to draining LNs. These organs are believed to have a critical role in the adaptive immune response to peripheral infections;therefore, understanding the cellular and molecular interactions that occur within LNs will provide insights that may be useful for improved vaccine development. Lymph-borne foreign matter entering LNs is captured and processed by antigen-presenting cells (APCs), which then present this material to B and T cells to elicit effector responses and long-lived immunological memory. The rules that determine how lymph-derived antigenic material is handled in LNs, especially in the context of ongoing infections, and what cells and molecules must interact to elicit a protective immune response (or fail to do so) are poorly understood. Why do some viruses (e.g. VSV in mice) induce a potent, multi-pronged protective immune response that eliminates the pathogen, while others (e.g. influenza) generate only transient protective immunity, and a few (e.g. HIV in humans) establish a chronic presence by continuously subverting and eventually exhausting the host's anti-viral defenses? The mechanisms behind these different outcomes are likely multi-factorial and depend upon differences in the way individual viruses interact with their hosts. To explore the dynamics of these interactions in living animals, all component projects of this Program will employ multi-photon intravital microscopy (MP-IVM) in intact LNs that will be offered in the Intravital Microscopy Core for time-and space-resolved visualization of the innate and adaptive immune response to lymph-borne virions. In Project 1, Dr. von Andrian will explore innate and adaptive immune responses to lymph-borne viral infections using VSV and several other viral pathogens. In Project 2. Drs. Sharpe and Wherry will visualize the effects of the negative costimulatory pathways, PD-1:PD-L1, PD-1:PD-L2 and PDL1 :B7.1 on antiviral immunity to influenza and LCMV. In Project 3. Dr. Carroll will dissect the role of complement and complement receptors in humoral immunity to influenza virus. Finally, in Project 4, Drs. Luster. Mempel and Tager will characterize the cellular dynamics and viral dissemination in HIV-infected lymph nodes of humanized mice. It is hoped that the mechanistic insights gained from these highly interactive and synergistic experiments will lead to new, evidence-based and knowledge-driven development strategies for anti-viral vaccines.